Fuell cell system and operation method of fuel cells

ABSTRACT

A fuel cell system of the invention includes: fuel cells that receive supplies of predetermined gases to generate electric power; and a humidifier that humidifies at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells. The fuel cell system further has: an exhaust water content detection module that detects an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier; and a regulation module that, in response to detection of the exhaust water content of not lower than a preset level, restricts the exhaust water content discharged downstream the humidifier. This technique of the invention ensures adequate humidification control.

TECHNICAL FIELD

The present invention relates to a fuel cell system including fuel cells that receive supplies of predetermined gases to generate electric power. More specifically the invention pertains to humidification control with the water content included in an exhaust gas from the fuel cells.

BACKGROUND ART

In a fuel cell system that receives supplies of hydrogen gas and the air as reactive gases and generates electric power through an electrochemical reaction of hydrogen with oxygen included in the air, humidification of the air supplied to the fuel cells is required to ensure sufficiently high power generation efficiency. One proposed technique applied to the fuel cell system uses a humidifier to humidify the air supplied to the fuel cells with the water content included in an exhaust gas produced on an oxygen electrode by the electrochemical reaction (see, for example, JP-A-2002-75418).

The fuel cell system disclosed in the cited patent document causes the air passing through the fuel cells (hereafter referred to as the exhaust gas) to be discharged outside via the humidifier. This fuel cell system has a first pressure regulator located between the fuel cells and the humidifier (that is, located upstream the humidifier) in the flow path of the exhaust gas and a second pressure regulator located downstream the humidifier. In response to a low humidity level of the air supplied to the fuel cells, the opening of the second pressure regulator is increased to lower the internal pressure of the humidifier. Such control is expected to vaporize the water content included in the exhaust gas to steam and enhance the humidification efficiency of the humidifier.

DISCLOSURE OF THE INVENTION

This prior art technique, however, does not take into account the exhaust gas discharged outside of the fuel cells and may thus cause undesirable deterioration of the humidification performance of the humidifier according to the quantity of the exhaust gas discharged outside. The water content in the exhaust gas is not wholly used by the humidifier, but part of the water content is discharged outside with the air as the exhaust gas. The water content included in the exhaust gas represents water produced in the course of power generation by the fuel cells and depends upon the amount of power generation. For example, the higher flow rate of the air supplied to the fuel cells for the increased amount of power generation results in the higher flow rate of the exhaust gas. In such circumstances, an increase in amount of steam may only raise the water content discharged outside as the exhaust gas. This may decrease the water content used for humidification and lower the humidification performance of the humidifier.

The object of the invention is thus to take into account the potential problem of the prior art technique that may decrease the humidification performance of the humidifier and to provide a fuel cell system that ensures adequate humidification control.

In order to attain at least part of the above and the other related objects, the present invention is directed to a fuel cell system including fuel cells that receive supplies of predetermined gases to generate electric power. The fuel cell system of the invention includes: a humidifier that is provided in a flow path of an exhaust gas from the fuel cells and humidifies at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; an exhaust water content detection module that detects an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier; and a regulation module that, in response to detection of the exhaust water content of not lower than a preset level, restricts the exhaust water content discharged downstream the humidifier.

The fuel cell system of the invention detects the exhaust water content discharged downstream the humidifier and restricts the exhaust water content in response to detection of the exhaust water content of not lower than the preset level. The technique of the invention controls the water content discharged downstream the humidifier and enables a greater portion of the water content included in the exhaust gas to be used for humidification of the supplied gas in the humidifier. This arrangement desirably prevents a decrease in humidification efficiency of the humidifier and thus ensures adequate humidification by the humidifier.

In one preferable application of the fuel cell system of the invention, the exhaust water content detection module detects the exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier, by measurement of a physical quantity affecting the exhaust water content.

The fuel cell system of this application measures the physical quantity affecting the exhaust water content and detects the exhaust water content based on the measured physical quantity. Namely the exhaust water content is obtainable indirectly from the physical quantity.

The physical quantity may be, for example, atmospheric pressure, outlet temperature of the exhaust gas from the fuel cells, or flow rate of the exhaust gas from the fuel cells. The physical quantity used in the invention is, however, not restricted to these examples but may be any physical quantity affecting the exhaust water content.

When the physical quantity is the atmospheric pressure, the exhaust water content of not lower than the preset level is detected under the condition that the measured atmospheric pressure is lower than a preset reference pressure. In the state of the low atmospheric pressure, it is expected to decrease the internal pressure of the humidifier and to increase the exhaust water content to or over the preset level. In this case, the fuel cell system controls the exhaust water content discharged downstream the humidifier. This arrangement ensures adequate humidification during operation of the fuel cell system even in the environment of the low atmospheric pressure.

When the physical quantity is the outlet temperature of the exhaust gas, the exhaust water content of not lower than the preset level is detected under the condition that the measured outlet temperature of the exhaust gas is higher than a preset reference temperature. In the state of the high temperature of the exhaust gas discharged from the fuel cells, it is expected to raise the water content included in the exhaust gas and to increase the exhaust water content to or over the preset level. In this case, the fuel cell system controls the water content discharged downstream the humidifier. This arrangement uses the generally measured temperature to readily detect the exhaust water content.

When the physical quantity is the flow rate of the exhaust gas, the exhaust water content of not lower than the preset level is detected under the condition that the measured flow rate of the exhaust gas is higher than a preset reference value. The high flow rate of the exhaust gas leads to the high flow velocity of the exhaust gas passing through the humidifier and results in insufficient humidification in the humidifier. In the state of the high flow rate of the exhaust gas, it is expected to increase the exhaust water content to or over the preset level. In this case, the fuel cell system controls the water content discharged downstream the humidifier. This arrangement ensures adequate humidification by the humidifier.

In one preferable embodiment of the fuel cell system of the invention, the regulation module has a downstream pressure regulator that is located downstream the humidifier in the flow path of the exhaust gas to regulate pressure of the exhaust gas and accordingly regulate internal pressure of the supplied gas in the fuel cells. The regulation module activates the downstream pressure regulator to perform pressure regulation for restriction of the exhaust water content discharged downstream the humidifier.

In the fuel cell system of this embodiment, the pressure regulation by the downstream pressure regulator is performed to restrict the flow rate of the exhaust water content. This arrangement ensures the relatively easy system construction by simply locating the pressure regulator downstream the humidifier.

In the fuel cell system of this embodiment, the regulation module further has an upstream pressure regulator that is located upstream the humidifier in the flow path of the exhaust gas to regulate the pressure of the exhaust gas and accordingly regulate the internal pressure of the supplied gas in the fuel cells. The regulation module activates the upstream pressure regulator to perform the pressure regulation, instead of the pressure regulation by the downstream pressure regulation, in response to detection of the exhaust water content of lower than the preset level.

In the fuel cell system of the embodiment with this additional structure, in the state of the low exhaust water content, the regulation module does not restrict the flow rate of the exhaust water content discharged downstream the humidifier but performs the pressure regulation by the upstream pressure regulator. The pressure regulation by the upstream pressure regulator located closer to the fuel cells desirably prevents a response delay and ensures the good controllability.

In one preferable structure of the above embodiment, the fuel cell system further has a humidification demand estimation module that estimates a humidification demand corresponding to a state of power generation by the fuel cells. When the estimated humidification demand is not higher than a specified level, the regulation module activates the upstream pressure regulator to perform the pressure regulation, irrespective of detection of the exhaust water content of lower than or not lower than the preset level.

In the fuel cell system of this structure, in response to a low humidification demand, the upstream pressure regulator is activated to perform the pressure regulation. Even in the case of an increase in exhaust water content to or over the preset level, when the humidification demand is relatively low, the pressure regulation by the upstream pressure regulator is performed with the priority placed on the controllability over the humidification efficiency of the humidifier. This arrangement ensures adequate humidification according to the requirements.

There is a fuel cell operation method corresponding to the fuel cell system described above. In the fuel cell system having fuel cells that receive supplies of predetermined gases to generate electric power and a humidifier that humidifies at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells, the operation method detects an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier, and in response to detection of the exhaust water content, restricts the exhaust water content discharged downstream the humidifier to be not lower than a preset level.

The present invention is also directed to another fuel cell system including fuel cells that receive supplies of predetermined gases to generate electric power, as well as to a corresponding fuel cell operation method. This fuel cell system further includes: a humidifier that is provided in a flow path of an exhaust gas from the fuel cells and humidifies at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; a decision module that identifies satisfaction or dissatisfaction of a condition for increasing an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier, based on a state quantity of the exhaust gas; and a pressure increase module that, upon satisfaction of the condition for increasing the exhaust water content, increases pressure of the exhaust gas in the humidifier to enhance a humidification efficiency of the humidifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the configuration of a fuel cell system embodying the invention;

FIG. 2 is a flowchart showing a pressure regulation process of a first embodiment executed in the fuel cell system of the invention;

FIG. 3 is a flowchart showing a pressure regulation process of a second embodiment executed in the fuel cell system of the invention; and

FIG. 4 is a flowchart showing a modified pressure regulation process, which includes decision based on a humidification demand, in addition to the pressure regulation process of the first embodiment.

BEST MODES OF CARRYING OUT THE INVENTION

Some modes of carrying out the invention are described below in the following sequence as preferred embodiments with reference to the accompanied drawings:

A. General Configuration of Fuel Cell System

B. Pressure Regulation Process in First Embodiment

C. Pressure Regulation Process in Second Embodiment

D. Modifications

A. General Configuration of Fuel Cell System

FIG. 1 schematically illustrates the configuration of a fuel cell system 10 embodying the invention. The fuel cell system 10 includes a stack of fuel cells or fuel cell stack 20 that receives supplies of hydrogen gas and the air as reactive gases and generates electric power through an electrochemical reaction of hydrogen with oxygen included in the air. The fuel cell system 10 is mounted on a vehicle (not shown) to work as a driving source for driving the vehicle with the electric power generated by the fuel cell stack 20. As illustrated, the fuel cell system 10 includes a hydrogen flow system 30 to feed the hydrogen gas to the fuel cell stack 20, an air flow system 40 to feed the air to the fuel cell stack 20, and a control unit 120 to control the constituents of the fuel cell system 10, in addition to the fuel cell stack 20.

The fuel cell stack 20 has a number of unit cells 21 that are laminated one another and individually have a hydrogen electrode (anode) and an oxygen electrode (cathode), and a pair of end plates 28 and 29 that are placed on the respective ends of the laminate of the unit cells 21. Each unit cell 21 has a separator, an anode, an electrolyte membrane, a cathode, and another separator laid one upon another in this order. The separators respectively have a flow path of the hydrogen gas and a flow path of the air. The flow paths of each fluid formed in the respective unit cells 21 join together and are connected to an inlet port of the fluid provided on the end plate 28. The hydrogen gas and the air supplied from the outside of the fuel cell stack 20 to the respective inlet ports are thus smoothly flowed to the individual unit cells 21. The end plate 28 also has an inlet port for a cooling medium, which is supplied from the outside of the fuel cell stack 20 to cool down the fuel cell stack 20.

The hydrogen gas supplied to the anodes of the respective unit cells 21 is converted to hydrogen ion by catalysis in catalyst layers of the respective anodes. The hydrogen ion passes through the electrolyte membranes to the cathodes to react with oxygen included in the air supplied to the cathodes. The unit cells 21 generate electric power through this electrochemical reaction. The fuel cell stack 20 has a plurality of such unit cells 21 connected in series to output high electric power. In the fuel cell system 10 of the embodiment, the electrolyte membranes used are solid polymer electrolyte membranes that have high operation performance in a predetermined range of humid environment.

The hydrogen flow system 30 includes a hydrogen tank 31 for storage of high-pressure hydrogen gas, a hydrogen circulation pump 32, and valves (not shown). The hydrogen gas after adjustment of the pressure and the flow rate by means of the valves is supplied to the fuel cell stack 20. The hydrogen content in the hydrogen gas supplied to the fuel cell stack 20 is mainly consumed by the above electrochemical reaction but may partly be unconsumed and discharged from the fuel cell stack 20. The hydrogen circulation pump 32 introduces the hydrogen gas discharged from the fuel cell stack 20 to the fuel cell stack 20 again for the effective use of the hydrogen content unconsumed by the electrochemical reaction and discharged from the fuel cell stack 20. The hydrogen gas fed to the fuel cell stack 20 is not restricted to the supply from the storage in the hydrogen tank 31. In one modified system, a fuel, such as methane or methanol, is reformed to produce hydrogen, which is then supplied to the fuel cell stack 20.

The air flow system 40 mainly has a supply line to feed the air to the fuel cell stack 20 and an exhaust line to introduce the air exhausted from the fuel cell stack 20 to an exhaust system 80.

The supply line has an atmospheric pressure sensor 47 with a built-in semiconductor gauge, an air cleaner 41 for removal of dirt and dust in the air, a hot-wire air flowmeter 42, an air compressor 43 including a motor as a driving source, an intercooler 44 for cooling down the air to increase the air density, a humidifier 48 for humidifying the supplied air, and supply conduits 45 and 46 for interconnecting these elements. The atmospheric pressure sensor 47, the air cleaner 41, the air flowmeter 42, the air compressor 43, the intercooler 44, and the humidifier 48 are arranged in this order along the flow of the air supply to the fuel cell stack 20. The outside air is taken in by the operation of the air compressor 43 and is fed to the fuel cell stack 20.

The outside air taken in by the operation of the air compressor 43 is cleaned by the air cleaner 41, passes through the air flowmeter 42, is compressed by the air compressor 43, is cooled down by the intercooler 44, and is humidified by the humidifier 48. The humidified air then flows through the supply conduit 46 connecting with the end plate 28 of the fuel cell stack 20 to be fed to the fuel cell stack 20.

In the fuel cell system 10 of the embodiment, the humidifier 48 used is a hollow fiber membrane humidifying device. The humidifier 48 has multiple hollow fiber membranes. A dried gas is flowed outside the hollow fiber membranes (this side is called the primary side), while a moist gas is flowed inside the hollow fiber membranes (this side is called the secondary side). The dried gas on the primary side is accordingly humidified with the moist gas on the secondary side. Each hollow fiber membrane has multiple microcapillaries going from the inside to the outside. The steam in the moist gas flowing on the secondary side is sucked out as the water content by capillarity. The sucked-out water content is supplied to the flow of dried gas on the primary side.

In the fuel cell system 10 of the embodiment, the primary side of the humidifier 48 is located on the supply line of the air flow system 40, whereas the secondary side of the humidifier 48 is located on the exhaust line. The air exhausted from the fuel cell stack 20 contains steam as the water content produced on the cathodes by the electrochemical reaction and is thus in the wet condition. The exhausted air in the wet condition is utilized to humidify the air supplied to the fuel cell stack 20.

On the supply line of the air flow system 40, the atmospheric pressure sensor 47 measures a pressure P1 as the atmospheric pressure of the outside air, and the air flowmeter 42 measures a flow rate ‘q’ of the air. The measured pressure P1 and flow rate ‘q’ are output to the control unit 120 and are used for control of the operations of the fuel cell system 10, for example, for regulation of the motor rotation speed of the air compressor 43 to adjust the supply of the air corresponding to a power generation demand.

The exhaust line has a temperature sensor 55 with a built-in thermistor, a semiconductor pressure sensor 56, a first pressure regulator 50 for regulating the pressure by the valve opening, the humidifier 48 (the secondary side), a second pressure regulator 58 of the same structure as that of the first pressure regulator 50, and exhaust conduits 51 and 52 for interconnecting these elements. The temperature sensor 55, the pressure sensor 56, the first pressure regulator 50, the humidifier 48, and the second pressure regulator 58 are arranged in this order along the flow of the air exhaust from the fuel cell stack 20. The air exhausted from the fuel cell stack 20 flows through the exhaust conduits 51 and 52 and is discharged outside.

The two pressure regulators 50 and 58 provided on the exhaust line regulate the pressure of the air at the outlet the fuel cell stack 20 and accordingly adjust the pressure of the air supplied to the fuel cell stack 20 to a predetermined range. Regulation of the outlet pressure (outlet pressure regulation) effectively prevents an excess load from being applied on the electrolyte membranes in the fuel cell stack 20 and enables the air supply to the fuel cell stack 20 at the adequate pressure level. Each of the pressure regulators 50 and 58 has a poppet valve element, which is moved back and forth to adjust the valve opening and accordingly regulate the pressure. The control unit 120 controls the rotational angle of a driving motor for the poppet valve element to adjust the valve opening.

On the exhaust line of the air flow system 40, the temperature sensor 55 measures a temperature T of the air exhaust from the fuel cell stack 20, and the pressure sensor 56 measures a pressure P2 of the air exhaust from the fuel cell stack 20. The measured temperature T and pressure P2 are output to the control unit 120 and are used for control of the operations of the fuel cell system 10, especially for a pressure regulation process to optimize humidification of the supplied air by the humidifier 48. The pressure regulation process here means the outlet pressure regulation executed by either of the two pressure regulators 50 and 58 according to preset conditions. In the outlet pressure regulation by the second pressure regulator 58 downstream the humidifier 48, the internal pressure of the humidifier 48 is regulated to the predetermined range to adjust the flow rate of the air that passes through the humidifier 48 and is discharged outside. Adjustment of the flow rate of the air discharged outside desirably enhances the humidification performance of the humidifier 48. The pressure regulation process will be described later in detail.

The control unit 120 includes a CPU, a ROM, a RAM, a timer, and input and output ports. A processing program for the pressure regulation, as well as diversity of other programs for controlling the operations of the whole fuel cell system 10 are stored in the ROM. The CPU loads these programs on the RAM and executes the processing according to the programs. The input port and the output port are respectively connected with various sensors and with various actuators. The control unit 120 receives signals from the various sensors, identifies the driving conditions of the vehicle, and controls the various actuators.

The control unit 120 receives inputs from the various sensors, for example, the pressure P1, the pressure P2, the temperature T, the air flow rate ‘q’, an output current A, an accelerator opening θ, and a vehicle speed V from the atmospheric pressure sensor 47, the pressure sensor 56, the temperature sensor 55, the air flowmeter 42, an ammeter 95 included in an output system 90 (described later), an accelerator position sensor (not shown), and a vehicle speed sensor (not shown). The control unit 120 then regulates the air compressor 43, the first pressure regulator 50, the second pressure regulator 58, the hydrogen circulation pump 32, and a pump 72 included in a cooling system 70 (described later) to drive the fuel cell system 10 according to the output demand (power generation demand).

In the fuel cell system 10 of this configuration, the fuel cell stack 20 is connected with the cooling system 70, the exhaust system 80, and the output system 90, as well as with the hydrogen flow system 30 and the air flow system 40.

The cooling system 70 includes a radiator 71, the pump 72, and conduits for interconnecting these elements and for connecting with the end plate 28 of the fuel cell stack 20. The electrochemical reaction proceeding in the fuel cell stack 20 is exothermic to raise the inner temperature of the fuel cell stack 20. A flow of cooling water (cooling medium) introduced into the fuel cell stack 20 to prevent the temperature rise is cooled down by the radiator 71 and is circulated by the pump 72.

A primary element of the exhaust system 80 is a muffler 81. The air flowing through the exhaust conduit 52 in the air flow system 40 is discharged to the outside air via the muffler 81. Nitrogen contained in the air may be transmitted through the electrolyte membranes to the anodes and may be concentrated through the circulation of the hydrogen gas in the hydrogen flow system 30. The exhaust system 80 is also connected with the hydrogen flow system 30, although not being specifically illustrated. The concentrated nitrogen is diluted with the air and is exhausted outside at preset timings.

The output system 90 includes an inverter 91, a drive motor 92 of the vehicle, a DC-DC converter 93, and a secondary battery 94. The electric power generated by the electrochemical reaction of the hydrogen gas and the air supplied to the fuel cell stack 20 is used via the inverter 91 to actuate the drive motor 92 of the vehicle. An excess electric power generated during cruise drive or under deceleration is regenerated by the motor 92 working as a generator and is accumulated into the secondary battery 94 via the DC-DC converter 93.

In the fuel cell system 10 of the embodiment, the atmospheric pressure sensor 47, the temperature sensor 55, the air flowmeter 42 (the air compressor 43), and the control unit 120 constitute the exhaust water content detection module in the claims of the invention. The first pressure regulator 50 and the second pressure regulator 58 are respectively equivalent to the upstream pressure regulator and the downstream pressure regulator in the claims of the invention. These pressure regulators 50 and 58 and the control unit 120 constitute the flow rate regulation module in the claims of the invention.

B. Pressure Regulation Process in First Embodiment

FIG. 2 is a flowchart showing a pressure regulation process of a first embodiment executed in the fuel cell system 10 described above. The pressure regulation process is executed by the control unit 120 after supply of the outside air by the air compressor 43 to the fuel cell stack 20 on activation of the fuel cell system 10. The first pressure regulator 50 and the second pressure regulator 58 are respectively set to a specified opening (default) and to a full open position, simultaneously with the activation of the fuel cell system 10. In the initial stage, the first pressure regulator 50 works to adjust the pressure of the air at the outlet the fuel cell stack 20 to a predetermined range.

On the start of the pressure regulation process, the control unit 120 inputs the pressure P1 measured by the atmospheric pressure sensor 47 (step S200) and determines whether the input pressure P1 is lower than a preset reference pressure α (step S215).

The atmospheric pressure is a physical quantity affecting the water content in the air flow discharged outside (exhaust water content). The current level of the exhaust water content and the variation in exhaust water content are estimated from the measurement result of the atmospheric pressure. This decision step S215 with regard to the atmospheric pressure is equivalent to computing the exhaust water content from the measured atmospheric pressure and determining whether the computed exhaust water content is not less than a preset level. The reference pressure α is set in advance as a standard value reflecting the exhaust water content and is stored in the ROM of the control unit 120. The reference pressure α set corresponding to the exhaust water content is used to identify whether the surrounding environment of the fuel cell system 10 is ‘highland’.

When the input pressure P1 is lower than the preset reference pressure α (step S215: yes), the surrounding environment of the fuel cell system 10 satisfies the high altitude condition that the atmospheric pressure is lower than the standard value and is thus identified as ‘highland’. In this case, the control unit 120 sets the first pressure regulator 50 to its full open position (step S230) and controls the second pressure regulator 58 to perform the outlet pressure regulation (step S240). When the outlet pressure regulation has been implemented by the first pressure regulator 50 at the initial stage, this step switches over the pressure regulator performing the outlet pressure regulation.

The outlet pressure regulation by the second pressure regulator 58 controls the outlet pressure of the air flow from the fuel cell stack 20 (eventually equivalent to the inlet pressure of the air flow) to the predetermined range. For example, when the current power generation level of the fuel cell stack 20 is excess over a power generation demand, the control unit 120 lowers the motor rotation speed of the air compressor 43 to decrease the flow rate of the air supply to the fuel cell stack 20. The decreased flow rate reduces the internal pressure of the exhaust conduit 51. The control unit 120 then detects this pressure fall in the exhaust conduit 51 based on the measurement result of the pressure P2 by the pressure sensor 56 and decreases the opening of the second pressure regulator 58 (that is, restricts the flow path) to raise the lowered pressure P2.

When the current power generation level of the fuel cell stack 20 is insufficient for the power generation demand, on the other hand, the control unit 120 raises the motor rotation speed of the air compressor 43 to increase the flow rate of the air supply to the fuel cell stack 20. The increased flow rate heightens the internal pressure of the exhaust conduit 51. The control unit 120 then detects this pressure rise in the exhaust conduit 51 based on the measurement result of the pressure P2 by the pressure sensor 56 and increases the opening of the second pressure regulator 58 (that is, opens the flow path) to lower the raised pressure P2.

The control unit 120 repeats this series of pressure regulation to keep the internal pressure of the fuel cell stack 20 at a substantially constant level. The outlet pressure regulation by the second pressure regulator 58 restricts the flow rate of the air discharged downstream the humidifier 48 and regulates the internal pressure of the humidifier 48 placed upstream the second pressure regulator 58 to a predetermined range of higher than the atmospheric pressure. After execution of this outlet pressure regulation for a specific time period, the processing flow goes to Next. This series of processing described above is repeated at preset timings. The second pressure regulator 58 is regulated to a pressure level determined by subtracting a pressure loss of the humidifier 48 from a target pressure value at the outlet of the fuel cell stack 20.

When the input pressure P1 is not lower than the preset reference pressure α (step S215: no), on the other hand, the surrounding environment of the fuel cell system 10 does not satisfy the high altitude condition that the atmospheric pressure is lower than the standard value and is thus identified as not ‘highland’. In this case, the control unit 120 sets the second pressure regulator 58 to its full open position (step S260) and controls the first pressure regulator 50 to perform the outlet pressure regulation (step S270). When the outlet pressure regulation has been implemented by the first pressure regulator 50 at the initial stage, this step continues the outlet pressure regulation by the first pressure regulator 50.

The outlet pressure regulation by the first pressure regulator 50 is performed in the similar manner to the outlet pressure regulation by the second pressure regulator 58 described above to keep the internal pressure of the fuel cell stack 20 at the substantially constant level. After execution of the outlet pressure regulation for a specific time period, the processing flow goes to Next. The above series of processing is repeated at preset timings. The outlet pressure regulation by the first pressure regulator 50 does not regulate the internal pressure of the humidifier 48 located downstream the first pressure regulator 50 but makes the internal pressure approximately equal to the atmospheric pressure.

Under the low atmospheric pressure to satisfy the high altitude condition, the pressure regulation process of the first embodiment uses the second pressure regulator 58 located downstream the humidifier 48 to regulate the internal pressure of the fuel cell stack 20 (to regulate the outlet pressure of the air flow). The control unit 120 decreases the opening of the second pressure regulator 58 to restrict the flow path and adjust the air pressure in the fuel cell stack 20 to the predetermined range of higher than the atmospheric pressure. This regulates the internal pressure of the humidifier 48 to a predetermined level of higher than the atmospheric pressure and enhances the humidification efficiency of the humidifier 48, compared with the efficiency under the low pressure condition (for example, under the atmospheric pressure at the highland). The enhanced humidification efficiency increases the rate of the water content used for humidification of the air flow passing through the humidifier 48. The enhanced humidification efficiency of the humidifier 48 to increase the rate of the water content used for humidification results in reducing the water content discharged with the exhaust gas.

The pressure regulation process of the first embodiment thus reduces the water content discharged outside the humidifier 48 (exhaust water content) in the environment of the high altitude condition, compared with the pressure regulation by the first pressure regulator 50 located upstream the humidifier 48. Namely the pressure regulation process of this embodiment effectively prevents a decrease in steam exchange efficiency in the humidifier 48 and ensures the sufficient air humidification even in the environment of the high altitude condition.

In regulation of the internal pressure of the fuel cell stack 20 by the first pressure regulator 50 located upstream the humidifier 48 in the environment of the high altitude condition, the internal pressure of the humidifier 48 (more specifically the pressure on the side of the moist air flow) drops to the atmospheric pressure level to worsen the humidification efficiency. The poor humidification efficiency reduces the water content used for humidification and increases the amount of steam included in the air flow passing through the humidifier 48. This may cause a large quantity of steam (water content) to be discharged outside with the air flow. The pressure regulation process of this embodiment, however, refers to the measurement result of the atmospheric pressure and identifies an increase in water content of the air flow exhausted from the fuel cell stack 20 (exhaust water content) to or over a preset level and reduces the exhaust water content taken out of the humidifier 48. This arrangement ensures the adequate humidification of the air flow with the balanced water content during the operations of the fuel cell system 10 even under the high altitude condition of the low atmospheric pressure, thus effectively preventing deterioration of the performance of the fuel cell stack 20.

The outlet pressure regulation is performed by the first pressure regulator 50 located upstream the humidifier 48 in the environment of no high altitude condition. In this case, the humidification performance of the humidifier 48 is not significantly lowered but ensures the adequate humidification of the air flow. The outlet pressure regulation by the pressure regulator at the position close to the air flow outlet of the fuel cell stack 20 (that is, by the first pressure regulator 50) enhances the response of pressure regulation. The pressure regulation process of this embodiment uses the measurement result of the atmospheric pressure by the atmospheric pressure sensor 47 located on the air intake side to identify satisfaction or dissatisfaction of the high altitude condition. One modified procedure may measure the pressure of the exhaust gas as one state quantity of the exhaust gas in the humidifier 48 and identify satisfaction of the high altitude condition based on the measured pressure of not higher than a specified level. Another possible modification may obtain altitude data from a car navigation system or another equivalent device to identify satisfaction or dissatisfaction of the high altitude condition. The state quantity of the exhaust gas is not restricted to the pressure of the exhaust gas but may be the temperature of the exhaust gas or the flow rate (flow velocity) of the exhaust gas as described below in another embodiment or a modified example.

The solid polymer electrolyte membrane is used as the electrolyte membrane in the fuel cell system 10 of the embodiment. The electrolyte membrane is, however, not restricted to this example but may be any other electrolyte membrane having the high operation performance in a predetermined range of humid environment. The pressure regulation process of the embodiment is applicable to attain the adequate humidification of the air flow in any fuel cell system including fuel cells with such electrolyte membranes and a humidifier for utilizing the water content in the exhaust gas to humidify the supplied air.

C. Pressure Regulation Process in Second Embodiment

The pressure regulation process of the first embodiment identifies an increase in exhaust water content discharged from the humidifier 48 to or over a preset level, on the basis of the measurement result of the atmospheric pressure. A pressure regulation process of a second embodiment, on the other hand, identifies an increase in exhaust water content to or over the preset level, on the basis of the measurement result of outlet temperature of the fuel cell stack 20. Namely the pressure regulation process of the second embodiment takes a different decision base for identification of the increase in exhaust water content from that of the pressure regulation process of the first embodiment but is otherwise similar to the pressure regulation process of the first embodiment (substantially similar outlet pressure regulation by either of the pressure regulators). The outlet pressure regulation performed in the second embodiment is thus only briefly mentioned. The hardware configuration for executing the pressure regulation process of the second embodiment is basically identical with the hardware configuration of the fuel cell system 10 shown in FIG. 1 and is thus not specifically described here.

FIG. 3 is a flowchart showing the pressure regulation process of the second embodiment executed in the fuel cell system 10. A processing program for the pressure regulation is stored in the ROM of the control unit 120. The CPU of the control unit 120 reads the processing program from the ROM and loads the processing program on the RAM to execute the pressure regulation process of the second embodiment.

On the start of the pressure regulation process, the control unit 120 inputs an outlet temperature T of the air flow from the fuel cell stack 20 measured by the temperature sensor 55 (step S300) and determines whether the outlet temperature T is higher than a preset reference temperature β (step S315).

Like the atmospheric pressure used as the basis of identification in the first embodiment, the outlet temperature of the air flow from the fuel cell stack 20 is a physical quantity affecting the exhaust water content. The current level of the exhaust water content and the variation in exhaust water content are estimated from the measurement result of the outlet temperature. This decision step S315 with regard to the outlet temperature is equivalent to computing the exhaust water content from the measured outlet temperature and determining whether the computed exhaust water content is not less than a preset level. The reference temperature β is set in advance as a standard value reflecting the exhaust water content and is stored in the ROM of the control unit 120.

When the outlet temperature T is higher than the preset reference temperature β (step S315: yes), an increase in steam (water content) of the air flow is expected. In this case, the control unit 120 sets the first pressure regulator 50 to its full open position (step S330), controls the second pressure regulator 58 to perform the outlet pressure regulation for a specific time period (step S340), and goes to Next. This series of processing is repeated at preset timings. The setting of the opening of the first pressure regulator 50 and the outlet pressure regulation by the second pressure regulator 58 are identical with the processing of steps S230 and S240 in the pressure regulation process of the first embodiment shown in the flowchart of FIG. 2.

When the outlet temperature T is not higher than the preset reference temperature β (step S315: no), on the other hand, no increase in steam (water content) of the air flow is expected. In this case, the control unit 120 sets the second pressure regulator 58 to its full open position (step S360), controls the first pressure regulator 50 to perform the outlet pressure regulation for a specific time period (step S370), and goes to Next. This series of processing is repeated at preset timings. The setting of the opening of the second pressure regulator 58 and the outlet pressure regulation by the first pressure regulator 50 are identical with the processing of steps S260 and S270 in the pressure regulation process of the first embodiment shown in the flowchart of FIG. 2.

Under the condition of the high outlet temperature T that suggests an increase in steam included in the air flow, the pressure regulation process of the second embodiment controls the second pressure regulator 58 to perform the outlet pressure regulation and restricts the flow rate of the air discharged downstream the humidifier 48. Like the pressure regulation process of the first embodiment, the pressure regulation process of the second embodiment controls the water content discharged outside the humidifier 48 (exhaust water content) and ensures the adequate humidification by the humidifier 48.

The physical quantity of the reactive gas, for example, the outlet temperature T of the air flow from the fuel cell stack 20, is generally measured for control of the fuel cell system 10. The use of this physical quantity for pressure regulation facilitates construction of the pressure regulation system. The flow rate ‘q’ of the air supply to the fuel cell stack 20 may be used, in place of the outlet temperature T, to identify the increase in exhaust water content to or over the preset level.

In such modification, the control unit 120 inputs the measurement result (flow rate ‘q’) of the air flowmeter 42 and compares the input flow rate ‘q’ with a preset reference value, instead of the processing of steps S300 and S315 in the pressure regulation process of FIG. 3. When the flow rate ‘q’ exceeds the preset reference value, the modified pressure regulation process goes to steps S330 and S340 to perform the outlet pressure regulation by the second pressure regulator 58. When the flow rate ‘q’ does not exceed the preset reference value, on the other hand, the modified pressure regulation process goes to steps S360 and S370 to perform the outlet pressure regulation by the first pressure regulator 50.

An increase in flow rate ‘q’ supplied per unit time over the preset reference value increases the flow velocity of the air discharged from the fuel cell stack 20 and lowers the humidification performance of the humidifier 48. This reference value is set corresponding to the exhaust water content like the reference pressure and the reference temperature used in the first and the second embodiments.

In this case, the outlet pressure regulation by the second pressure regulator 58 located downstream the humidifier 48 is performed to restrict the flow rate of the air discharged downstream the humidifier 48. This outlet pressure regulation by the second pressure regulator 58 decreases the water content discharged outside the humidifier 48 (exhaust water content), compared with the outlet pressure regulation by the first pressure regulator 50. The flow rate of the supplied air (volume of air flow) may be estimated from the motor rotation speed of the air compressor 43.

D. Modifications

The embodiments discussed above are to be considered in all aspects as illustrative and not restrictive. There may be many modifications, changes, and alterations without departing from the scope or spirit of the main characteristics of the present invention. The pressure regulation processes of the first and the second embodiments described above compute the exhaust water content from the measured physical quantity, for example, the measured atmospheric pressure or the measured outlet temperature, to set the reference pressure α or the reference temperature β. Computation of the exhaust water content is, however, not essential. A physical quantity experimentally or otherwise correlated to the exhaust water content may be used to set the reference pressure or the reference temperature, while the exhaust water content is kept undetermined.

The pressure regulation processes of the first and the second embodiments restrict the flow rate of the air discharged downstream the humidifier 48, based on the comparison between the measurement result and the preset reference value. One possible modification may increase the restriction degree of the flow rate with an increase in exhaust water content (linear control). For example, the restriction degree of the flow rate is determined corresponding to the given atmospheric pressure by referring to a predetermined correlation map.

The pressure regulation process described above uses only one of the atmospheric pressure, the outlet temperature, and the flow rate of the supplied air as the basis of identification of the increase in exhaust water content. One modified procedure may input all these physical quantities and perform the outlet pressure regulation by the pressure regulator located downstream the humidifier when any one of the input physical quantities satisfies the preset condition.

The pressure regulation processes of the first and the second embodiments specify one of the two pressure regulators located upstream and downstream the humidifier to be used for execution of the outlet pressure regulation, on the basis of the measurement result of the atmospheric pressure or the outlet temperature (or the flow rate of the supplied air). The pressure regulator to be used for execution of the outlet pressure regulation may be specified by additionally taking into account a humidification demand required for the adequate power generation by the fuel cell stack 20.

FIG. 4 is a flowchart showing a modified pressure regulation process, which includes decision based on a humidification demand, in addition to the pressure regulation process of the first embodiment. This pressure regulation process is executed by the control unit 120, like the pressure regulation process of the first embodiment shown in the flowchart of FIG. 2. The like step numbers denote the like processing steps to those in the pressure regulation process of the first embodiment.

On the start of the modified pressure regulation process, the control unit 120 inputs the pressure P1 as the measured atmospheric pressure (step S200) and determines whether the atmospheric pressure is lower than the standard value (reference pressure α) (step S215). When the atmospheric pressure is not lower than the standard value (step S215: no), the control unit 120 sets the second pressure regulator 58 to its full open position (step S260), controls the first pressure regulator 50 to perform the outlet pressure regulation for a specific time period (step S270), and goes to Next. Namely the pressure regulator 50 located upstream the humidifier 48 is controlled to execute the outlet pressure regulation for the specific time period. This series of processing is repeated at preset timings.

When the atmospheric pressure is lower than the standard value (step S215: yes), on the other hand, the control unit 120 computes a humidification demand from the measurement values of the various sensors (step S420).

The concrete procedure of the computation first calculates the quantity of the supplied air from the measurement value of the air flowmeter 42, the consumption of oxygen for the electrochemical reaction and the quantity of water produced by the electrochemical reaction from the measurement value of the ammeter 95, and the flow rate of the exhausted air flow from the measurement values of the temperature sensor 55, the pressure sensor 56, and the openings of the pressure regulators 50 and 58. The procedure then computes the current water content included in the air flow in the fuel cell stack 20 and refers to a correlation map of the water content to the amount of power generation to determine the humidification demand required for the adequate power generation corresponding to the computed water content.

The control unit 120 then determines whether the computed humidification demand is greater than a preset value γ (step S425).

When the computed humidification demand is greater than the preset value γ (step S425: yes), the control unit 120 sets the first pressure regulator 50 to its full open position (step S230), controls the second pressure regulator 58 to perform the outlet pressure regulation for a specific time period (step S240), and goes to Next. Namely the pressure regulator 58 located downstream the humidifier 48 is controlled to execute the outlet pressure regulation for the specific time period. This series of processing is repeated at preset timings.

When the computed humidification demand is not greater than the preset value γ (step S425: no), on the other hand, the control unit 120 sets the second pressure regulator 58 to its full open position (step S260), controls the first pressure regulator 50 to perform the outlet pressure regulation for a specific time period (step S270), and goes to Next. Namely the pressure regulator 50 located upstream the humidifier 48 is controlled to execute the outlet pressure regulation for the specific time period. This series of processing is repeated at preset timings.

The modified pressure regulation process controls the pressure regulator 50 located upstream the humidifier 48 to perform the outlet pressure regulation even under the high altitude condition of the low atmospheric pressure, when the humidification demand is not greater than the preset value. Even in the case of an increase in exhaust water content to or over the preset level, there is no high necessity of significant humidification when the humidification demand currently required in the fuel cell stack 20 is not greater than the preset value. In such cases, priority is placed on the response (controllability) of the outlet pressure of the air flow from the fuel cell stack 20. The outlet pressure regulation is thus performed by the first pressure regulator 50 at the position close to the outlet of the air flow from the fuel cell stack 20. This enables regulation of the outlet pressure with high response.

In the configuration of the embodiment, the pressure regulators are provided upstream and downstream the humidifier, and the pressure regulator used for regulation of the outlet pressure of the air flow from the fuel cell stack is switched over according to the predetermined condition. Such switchover of the pressure regulator used for the outlet pressure regulation is, however, not essential. For example, the outlet pressure regulation is unconditionally performed by the pressure regulator located upstream the humidifier. In response to a potential increase in exhaust water content to or over the preset level, which is suggested by a decrease in atmospheric pressure, an increase in outlet temperature, or an increase in flow velocity of the exhausted air flow, the pressure regulation may decrease the opening of the pressure regulator located downstream the humidifier (to restrict the flow path). This decreases the water content discharged outside and prevents deterioration of the humidification performance of the humidifier, while facilitating control of the two pressure regulators. 

1. A fuel cell system including fuel cells that receive supplies of predetermined gases to generate electric power, said fuel cell system further comprising: a humidifier that is provided in a flow path of an exhaust gas from the fuel cells and humidifies at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; an exhaust water content detection module that detects an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier; and a regulation module that, in response to detection of the exhaust water content of not lower than a preset level, restricts the exhaust water content discharged downstream the humidifier.
 2. A fuel cell system in accordance with claim 1, wherein the exhaust water content detection module detects the exhaust water content by measurement of a physical quantity affecting the exhaust water content.
 3. A fuel cell system in accordance with claim 2, wherein the exhaust water content detection module is a pressure sensor that measures atmospheric pressure as the physical quantity, and the regulation module determines that the exhaust water content is not lower than the preset level, when the measured atmospheric pressure is not higher than a preset reference pressure.
 4. A fuel cell system in accordance with claim 2, wherein the exhaust water content detection module is a temperature sensor that measures temperature of the exhaust gas at an outlet of the fuel cells as the physical quantity, and the regulation module determines that the exhaust water content is not lower than the preset level, when the measured temperature is not lower than a preset reference temperature.
 5. A fuel cell system in accordance with claim 2, wherein the exhaust water content detection module is a flow rate sensor that measures a flow rate of the exhaust gas from the fuel cells as the physical quantity, and the regulation module determines that the exhaust water content is not lower than the preset level, when the measured flow rate is not lower than a preset reference value.
 6. A fuel cell system in accordance with claim 2, wherein the regulation module comprises: a downstream pressure regulator that is located downstream the humidifier in the flow path of the exhaust gas to regulate pressure of the exhaust gas and accordingly regulate internal pressure of the supplied gas in the fuel cells, and the regulation module activates the downstream pressure regulator to perform pressure regulation for restriction of the exhaust water content discharged downstream the humidifier.
 7. A fuel cell system in accordance with claim 6, wherein the regulation module further comprises: an upstream pressure regulator that is located upstream the humidifier in the flow path of the exhaust gas to regulate the pressure of the exhaust gas and accordingly regulate the internal pressure of the supplied gas in the fuel cells, and the regulation module activates the upstream pressure regulator to perform the pressure regulation, instead of the pressure regulation by the downstream pressure regulation, in response to detection of the exhaust water content of lower than the preset level.
 8. A fuel cell system in accordance with claim 7, said fuel cell system further comprising: a humidification demand estimation module that estimates a humidification demand corresponding to a state of power generation by the fuel cells, wherein when the estimated humidification demand is not higher than a specified level, the regulation module activates the upstream pressure regulator to perform the pressure regulation, irrespective of detection of the exhaust water content of lower than or not lower than the preset level.
 9. A fuel cell system including fuel cells that receive supplies of predetermined gases to generate electric power, said fuel cell system further comprising: a humidifier that is provided in a flow path of an exhaust gas from the fuel cells and humidifies at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; a decision module that identifies satisfaction or dissatisfaction of a condition for increasing an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier, based on a state quantity of the exhaust gas; and a pressure increase module that, upon satisfaction of the condition for increasing the exhaust water content, increases pressure of the exhaust gas in the humidifier to enhance a humidification efficiency of the humidifier.
 10. A fuel cell system in accordance with claim 9, wherein the decision module identifies satisfaction or dissatisfaction of the condition, based on any one of pressure, temperature, and flow rate of the exhaust gas in the humidifier.
 11. A fuel cell system in accordance with claim 9, wherein the pressure increase module has a pressure regulator located downstream the humidifier and controls the pressure regulator to increase the pressure.
 12. An operation method of fuel cells that receive supplies of predetermined gases to generate electric power, said operation method comprising the steps of: activating a humidifier that is provided in a flow path of an exhaust gas from the fuel cells to humidify at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; detecting an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier; and in response to detection of the exhaust water content, restricting the exhaust water content discharged downstream the humidifier to be not lower than a preset level.
 13. An operation method of fuel cells that receive supplies of predetermined gases to generate electric power, said operation method comprising the steps of: activating a humidifier that is provided in a flow path of an exhaust gas from the fuel cells to humidify at least one of the predetermined gases supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; identifying satisfaction or dissatisfaction of a condition for increasing an exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier, based on a state quantity of the exhaust gas; and upon satisfaction of the condition for increasing the exhaust water content, increasing pressure of the exhaust gas in the humidifier to enhance a humidification efficiency of the humidifier.
 14. A fuel cell system including fuel cells that receive a supply of hydrogen gas as a fuel gas and a supply of the air as an oxidizing gas to generate electric power, said fuel cell system further comprising: a humidifier that is provided in a flow path of an exhaust gas in an air system from the fuel cells and humidifies the air supplied to the fuel cells with water content included in the exhaust gas discharged from the fuel cells; a sensor that detects at least one of atmospheric pressure, temperature of the exhaust gas, and flow rate of the exhaust gas as a physical quantity corresponding to a variation in exhaust water content, which is included in the exhaust gas and is discharged downstream the humidifier; and a pressure regulator that is located downstream the humidifier and, in response to detection of the exhaust water content of not lower than a preset level by the sensor, restricts the flow path of the exhaust gas to increase pressure of the exhaust gas in the humidifier. 